1. Field of the Invention
The invention relates to a malfunction diagnostic device for an exhaust gas control device.
2. Description of the Related Art
In an internal combustion engine, for example, an automobile diesel engine, an exhaust gas control device that purifies the exhaust gas is provided in an exhaust system. Examples of this type of exhaust gas control device include a PM filter that traps particulate matter (hereinafter, referred to as “PM” where appropriate) which is composed mainly of soot, a catalytic converter that supports a catalyst which purifies the exhaust gas by removing hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), etc. from the exhaust gas. Whether these exhaust gas control devices malfunction may be determined based on parameters that indicate the exhaust gas purification performance of the exhaust gas control devices. More specifically, if the parameter that indicates the exhaust gas purification performance of the exhaust gas control device crosses a predetermined threshold, it is determined that the exhaust gas control device malfunctions.
As a parameter that indicates the exhaust gas purification performance of an exhaust gas control device, for example, the difference in pressure between the upstream side and the downstream side of the exhaust gas control device provided in an exhaust system may be used, as described in Japanese Patent Application Publication No. 2003-155920 (JP-A-2003-155920). According to JP-A-2003-155920, a threshold used to determine whether clogging of the exhaust gas control device has occurred (hereinafter, referred to as “clogging threshold”) and a threshold used to determine whether perforation of the exhaust gas control device has occurred (hereinafter, referred to as “perforation threshold”) are calculated based on the engine operating state, for example, the engine speed and the fuel injection amount. If clogging of the exhaust gas control device has occurred and the exhaust gas purification performance thereof has deteriorated, the above-described pressure difference increases due to the clogging. Therefore, if the pressure difference is equal to or greater than the clogging threshold, it is determined that clogging of the exhaust gas control device has occurred. If perforation of the exhaust gas control device has occurred due to, for example, melting and the exhaust gas purification performance thereof has deteriorated, the pressure difference decreases due to the perforation. Therefore, if the pressure difference is less than the perforation threshold, it is determined that perforation of the exhaust gas control device has occurred.
If it is determined that a malfunction, for example, clogging or perforation of the exhaust gas control device has occurred, the exhaust gas control device is replaced with a new one.
As the accumulated hours of use of the exhaust gas control device increase, the exhaust gas control device deteriorates. As the degree of deterioration increases, a malfunction is more likely to occur. If the degree of deterioration of the exhaust gas control device falls within a certain range of degree of deterioration, the exhaust gas control device is able to purify the exhaust gas. The threshold, which is used to determine whether the exhaust gas control device malfunctions, may be set to a value that corresponds to the upper limit of the range so that a malfunction of the exhaust gas control device is reliably detected. In this case, if it is determined that the exhaust gas control device malfunctions based on the fact that the parameter that indicates the exhaust gas purification performance of the exhaust gas control device crosses the threshold before the degree of deterioration of the exhaust gas control device reaches the upper limit, the determination is regarded as being appropriate.
However, if the threshold is set in the above-described manner, the following inconvenience occurs when the degree of deterioration of the exhaust gas control device is low, for example, when the exhaust gas control device is nearly new When the degree of deterioration of the exhaust gas control device is low, a malfunction of the exhaust gas control device is unlikely to occur. Therefore, if the threshold that is set in the above-described manner is used to determine whether the exhaust gas control device malfunctions, the likelihood that the exhaust gas control device is determined to be malfunctioning is significantly increased. Accordingly, even if it is determined that the exhaust gas control device malfunctions based on the fact that the parameter crosses the threshold, a serious malfunction that may cause a problem has actually not occurred. That is, an erroneous determination is made. In this case, the exhaust gas control device is replaced with a new one based on the erroneous determination that the malfunction has occurred, before the degree of deterioration of the exhaust gas control device reaches the upper limit. As a result, the exhaust gas control device is replaced with a new one unnecessarily early.